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Entanglement and time

  1. May 12, 2006 #1
    I've seen little reference material on my question, except the occasional statement that "that is not the case", but no explanation as to why it is not the case.

    I suppose this is a common question asked by the layman, but that's me so, I'm asking.

    If entangled particles are able to respond to their partners in what "appears to be a faster than light" (non-local) means and if such particles are traveling at the speed of light, then from the particle's viewpoint, there has been no passage of time. So from their viewpoint, wouldn't the time at which they were first entangled, and the time at which we measure or alter a complimentary feature, be the same time? Therefore, no need for FTL communication or backwards in time communication. It wouldn't matter when you did the measurement, the time at creation and at measurement would be the same.

    I'm sure I'm just missing some piece of what is a logically difficult subject.

    Have wave function or entanglement experiments with entanglement on particles traveling significantly less than C been done? If so, I imagine that would invalidate any connection between relativistic speeds and entanglement.

    Thanks for humoring the new guy.

    Daryl
     
    Last edited: May 12, 2006
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  3. May 12, 2006 #2

    ZapperZ

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    I've always wanted to respond to questions like this, but never had the time (or patience) till now. I think DrChinese can correct me if I make any mistakes here.

    Consider this classical scenario. You have a mass that initially has no angular momentum. Suddenly, it explodes into two pieces. The pieces fly apart in opposite direction.

    At a later time, Piece A is very far away from Piece B. I captured Piece A and try to meausure its angular momentum (i.e. is it spinning, wobbling, etc). At the instant that I measure its angular momentum, I automatically know the angular momentum of Piece B also! This is via invoking the conservation of angular momentum. There's nothing exotic, spectacular, or even strange here. Nothing is being transfered between the two.

    Now, how is this different than the quantum entanglement case? It is (i) the introduction of the superposition of the entangled states and (ii) the measurement of non-commuting observables, that make this very unlike the classical case. It is why I often tell people who are trying to understand entanglement that you cannot simply get this without understanding the foundation of QM which involves both the concept of superposition and commuting/non-commuting observables.

    In the classical case above, after the explosion, each piece has some definite angular momentum, even before we measure it. We just don't know what it is, other than what they should add up to. In the quantum entanglement case, ALL the possible states are there in the mixture. This is what we mean by superposition. It is only after you understand what superposition means can you handle entanglement issues. So you do not have any idea what will come up upon a measurement till you measure it. When that occurs, then voila, the same "conservation" issue will kick in as in the classical case.

    However, this isn't the end of it. I could, for instance modify slighly my measurement. If I measure the direction of the angular momentum of Piece A to be along some direction in space, and then I meausre the direction of the angullar momentum of Piece B along another direction, then classical mechanics will tell me how they will both related simply via conservation laws. In quantum entanglement, this can differ depending on the angular different between the two measurement direction. What you are doing here is trying to measure the "non-commuting" component of the angular momentum. QM has a slightly different set of results for such a system. These difference have been observed numerous times in many experiments (I have seen ZERO experiments that do not agree with QM predictions).

    What I'm trying to get across here is that physics will look VERY strange if one tries to understand it only in bits and pieces. The whole issue of quantum entanglement, strangely enough, isn't about the entanglement itself. You've already seen how a similar classical system doesn't bother anyone. It is the phenomenon of superposition (the same one that resulted in the Schrodinger Cat experiment) that actually is the one responsible for making it non-classical. But most people miss this.

    Zz.
     
  4. May 12, 2006 #3
    The mathematically rigorous treatment of something that is profoundly mysterious doesn’t make it any less mysterious. The problem can be stated with the simplest mathematics possible, and no amount of mathematical sophistication can improve our understanding of what is going on in reality (as against in the formulae).
    Please stick to the common usage of technical terms. A mixture and a superposition are two very different things. We had a long thread on this.
    Congratulations if you can see the whole picture, and twice congratulations if this makes the physics appear to you any less strange.
     
    Last edited by a moderator: May 12, 2006
  5. May 12, 2006 #4
    Great responses but what about the time factor? What about Aspect's ERP experiments in which changes to one particle manifest themselves in the partner at what would be FTL speeds if they were "communicating"? Can relativity possibly play a part here? I've heard "no" but not "why" time is not a factor.

    Daryl
     
  6. May 12, 2006 #5

    ZapperZ

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    Please don't turn this into another of your philosophical mumbo jumbo. I do not care for it and you can save it for your discussion in the Philosophy forum.

    What exactly did I describe here that is wrong? And I'm not talking about "mixture" as in mix and pure states either.

    And you seem to know what "reality" seems to be and that if it is strange to you, it must also be "strange" to me. I do not find quantum superposition as "strange" since I observe its consequences often. I can show you food that you find "strange", but not to others who eat it often. So if you want to be picky about my usage of the word "mixture", I can play the same game with you and pick on your usage of the word "strange", "reality", etc.

    And this criticism of mathematics being insufficient for our "understanding" of anything is seriously bogus. What is the alternative? Having-waving arguments the way you have been practicing?

    Zz.
     
  7. May 12, 2006 #6

    ZapperZ

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    What "time" factor? These particles aren't "communicating" the same way the two classical pieces aren't communicating.

    Zz.
     
  8. May 12, 2006 #7
    I'm beginning to really like this forum. This is a lot of fun!

    Uh... what about the ERP, entanglement time thing....?
     
  9. May 12, 2006 #8
    NO they are NOT communicating. But if the complimentary factors were set at the time of entanglement, then there would be no mystery, but it "appears" to change FTL later, unless the time of the change is the same as the time of the entanglement, then due to relatively, no mystery.

    My question is, why can't relativity explain the why they appear to be connected non-locally later?

    I thought my question was pretty straight forward. (Of course they don't "communicate", how could you even suggest that I really thought so? ... unless they have really really really tiny microphones...hmmm.....)

    Daryl
     
    Last edited: May 12, 2006
  10. May 12, 2006 #9

    ZapperZ

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    I'd answer this if I knew what exactly you are saying here. Maybe someone else can.

    Zz.
     
  11. May 12, 2006 #10
    Wow, I thought all you guys were really smart, but let me try to explain my dumb question. Please don't accuse me of "thinking" in layman terms I use.

    Think about Aspect's ERP experiments. You change a complimentary of a particle, and "like magic" the other particle can be measured to reflect the complimentary "in less time than it would have taken to send a "signal" to between the particles. (Now just for clarity, I don't think they are talking to each other)

    While their complementary parameters could be set at the time of entanglement, obviously something is going on that can't be explained by classical means. (I assumed everyone knew about this stuff)

    My question is: If the particles are traveling at C, then for the particles no time has passed, so frail logic suggests, that the time at which they were entangled would be the same (for the particles) as the time of any alteration or measurement. If this time is the same then why can't they still be locally connected at the point of entanglement? If I change a complimentary on a particle after they are separated, can the particles still be locally connected at the point of entanglement from "their" perspective. (Please I know they don't have a real perspective, they are not "alive".... at least as far as we know. Just kidding, jeez).

    In a nutshell the entire FTL path of the entangled particles should from their perspective be the same point in time. So if I change a complimentary, no time has passed for them, why can't they still align themselves at the point of entanglement, since all time is the same for them.

    Now, I have read references saying this is NOT so, but I've not read what the flaw in the "logic" is?

    Please don't treat me as dumb, I'm not a math wizard and it's hard to articulate in this quick notes, but I'm probably better informed than I come across here. I can't help thinking about my own thought experiments, I was hoping some folks here could help me sort things out.

    Thanks, and lighten up guys. It's Friday!!!!!!

    Daryl
     
  12. May 12, 2006 #11

    ZapperZ

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    Because the measurement is done in a lab frame, not in the photon frame (or the particle-in-question frame). These two are separated by a large distance.

    Why? The photon is not doing the measurement.

    What is "change a complimentary"?

    Zz.
     
  13. May 12, 2006 #12
    I guess I can't re-word my question much differently.

    I assumed mentioning the ERP Aspect experiments would explain.

    Very simply. Two entangled particles head off in different directions, I force a change to something such as spin, or polarization, and presto the partner changes quicker than any "signal" could be sent at light speed. Obviously the results of this experiment prove that they are not connected by some unobserved EM signal that causes one particle to change in response to another.

    I've read a LOT, but as far as I know, and I'm pretty sure I'm right, there is no known explanation as to how this can be. It's pretty much akin to magic from the classical human perspective.

    My musings are... We could claim, rightly or wrongly that if we did nothing but measure spin or polarization on these particles then we could say we were just measuring pre-existing complimentary parameters.

    But since we can change the parameters of one and the other automatically responds, then we are left to either assume magic (not an altogether irrational assumption) or more likely, there is something that we don't understand.

    Now, backup to the point of entanglement. The particles are one. Obviously at this point we can assume any changes will effect the subsequent entangled particles, because they are one and the same. No mystery there. But if time does not pass for these particles then why can't we assume that our later alteration of one particle happens, in some way, at the same time as when the particles were entangled? Therefore, no mystery since at the time of entanglement, they were connected. No backward communication in time (as has been proposed in the past, albeit not taken seriously) because for the particles the moment of entanglement is not in the past.

    Now this is not any less strange than the mystery itself, but if we assumed that relativity could somehow allow the particles to still preserve some aspect of the "time" at which they entangled, it would provide some sort of logic to this mystery.

    Now, I'm not saying this is what I think happens. I'm saying this is a thought I had, and I've read that the relativity issue is not the answer. I'm just looking for some basic reason why that it is not the answer.

    If this still confusing, then I don't know what more I can add.

    Thanks guys.

    Daryl
     
  14. May 12, 2006 #13

    ZapperZ

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    First of all, in case this isn't clear, I am very well-aware of Aspect EPR experiment, and in fact, there's a whole slew of other newer experiments that do not have the same loophole as the original Aspect experiments. You may want to try looking at those.

    Sorry, but you don't force any change. You just measure the component of the angular momentum, and then you vary this angle over other measurements, one at a time. You then correlated the measurements of both particles.

    There's no explanation for conservation of energy/mass and conservation of momentum (linear and angular) either other than to invoke the symmetry principles. And one can easily say that there's no "explanation" for those symmetry principles either.

    Here's where the missing understanding of the mathematics are getting to you. Remember what all of these "thought" experiments are trying to do - they are trying to convey what the mathematics actually is trying to say. So if you do not understand, or are not aware of the mathematics, you are missing the whole source of the story. You then have to rely on 2nd or even 3rd hand news, and we all know how something like that can give a wrong story or idea. If you cannot realize that this is all based on a generic mathematical description, akin to

    |psi> = a|u1,v2> + b|u2,v1>

    then you would end up making statements invoking things that do not actually occur, such as "... we can change the parameters of one and the other automatically responds.... ". All of the parameters are there! Your measurement only happens to selected one of the available values, and when you do that, the partner is automatically selected because u and v are not separable (as in mathematically).

    Both the Schrodinger Cat and the EPR experiments are illustrations of what the mathematics are saying. This is crucial to keep in mind. If you are doing a translation of something, think of what is the most "accurate" means of doing a translation - directly from the original source, or a translation of a translation? In our case, the original source is the mathematics. If you are trying to re-explain EPR in a different way, you need to start at the source, or else you are basing your understanding on 2nd and 3rd hand "translations".

    Zz.
     
  15. May 12, 2006 #14
    Daryl
    I’ll grant you yours is a simple enough question, But don’t expect a simple statement to resolve the issue for you in a simple statement or answer.

    What you need is just a simple understanding on your part of the role relativity plays.

    First be sure your clear on the difference between “Space-Like Separated” and Time- Like Separated” in relativistic terms. (Outside vs. Inside the cone of light).

    The experiment is not testing or comparing data at some reference frame moving with individual particles of light. They are tests of events at real locations carefully designed to be relativistic Space-Like Separated events in “space-time”.

    Although your thought of time for the light within its own reference frame “forever” remaining the same is cute, -- that ‘time’ is also stuck on a specific line though spatial space that never reaches the other test sight. And even worse “forever” comes to an end when the light particle is absorbed by the detector living in a reference frame where time does move. It is in a reference frame of measurements that we see the issue, not that of the photon.

    So it is requirements and demands of relativity that actually defines the physics of the problem. Some additional understanding or explanation needs to be added to resolve the issue.

    On paper many interpretations can resolve the issue, Classical, BM, QM, or Multi-dimensions such as String, M, MWI, plus I’m sure many more. So far no one idea has been convincing, some even have “proofs” against them (Mine and Einstein’s favorite included).

    You should be able to see from what you already know that relativity needs something from some interpretation to complete a solution to the issue it can only define.

    Think of it like this – at best your timeless photon can cast a ‘shadow’ timelessly across other reference frames but likely only in a specific direction and certainly the ‘shadow’ needs to follow the rules of the Space-Like separation of the light cone when measure in those 'real' referance frames.
    That is, the no change in time applies only within your photon not to the other frames where the experiment is taking place and the photon only visits.

    Help any?
     
  16. May 12, 2006 #15
    You said : "Your measurement only happens to select one of the available values, and when you do that, the partner is automatically selected because u and v are not separable (as in mathematically)."

    But the particles are separated in space (if not in time), and while the math may make sense, the non-local nature of the experiment does not in the classical sense.

    We are left with either trying to make sense of it, or just claim there is no mystery because that's the way it is. (Shut up and calculate.) I would suspect that the quantum fuzziness does play a role, i.e. they both inherently have all values, but like all great magic acts, we are left with saying "How does he do that?". When we find out it spoils the illusion and the fun is gone. In this case I suspect the "how" is another mystery. And yes I do look at things from the classic viewpoint. What else can I do as a macroscopic classic being?

    I would like the references to any experiments akin to Aspect's if you have links or books I should read. Thanks.

    Daryl

    Daryl
     
  17. May 12, 2006 #16
    RandallB.

    Thank you. You have hit exactly on what I was getting at. You are right of course that we cannot look at space and time separately but rather as "spacetime". That may be explanation enough.

    But there still remains the issue of how it can happen at all. Again we are trying to understand it in macroscopic terms, but then what else can we do? I suppose for all intents and purposes the entangled particles are not separate particles but separate manifestations of the same particle. Perhaps it exists in whole in a separate dimension, in which case we are probably doomed and we should shut up and calculate.

    Myself, I would rather look for the hidden dimension, or whatever we come up with next, I don't calculate well, and as anyone will tell you - I never shut up.

    Thanks. You've given me food for thought. I'll have to think the problem over again as a "spacetime" issue. That's the fun part for me.

    Daryl
     
  18. May 12, 2006 #17

    ZapperZ

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    And why is that a problem? Since when does nature have to conform to our "classical sense"?

    To me, it makes no difference because I hate to force nature to conform to what I think makes sense. If you consider this carefully, what didn't use to make sense to you before, is making sense to you now AFTER you learn it a bit more. Requiring something to make sense sometime is nonsensical.

    Or maybe look at it as another part of our world that we are not familiar with. What is wrong with that?

    This has nothing to do, really, with EPR. It has everything to do with QM and why many people do not find it easy to understand it. Your question could easily be replaced with any generic QM phenomena. It was why I wrote this essay quite a while back, because I see this often when people try to "intelectualize" what they learn superficially and try to come up with their own explanation.

    http://www.physicspost.com/science-article-208.html

    QM is different because the world we tried to prod and understand is different, and there's enough hints to indicate that the concepts we use simply do not fit very cleanly in that world. I see no problems in accepting that. I see problems, however, in forcing something to conform with pre-conceived ideas on what should "make sense".

    Zz.
     
  19. May 12, 2006 #18
    ZapperZ,

    I reviewed your article: Why is Quantum Mechanics SO difficult?

    "You cannot use your existing understanding of the universe to try to understand the various concepts of QM. There is a discontinuity between the two. It is only via the mathematical continuity of the description can there be a smooth transition to build upon. Without this, QM will not make "sense".

    And while you are right, it doesn't erase the discontinuity. It's sort of like Galileo, "the truth is still there." Early astronomers could explain the motion of the planets with faulty logic (except Mercury), but it didn't mean that they erased any mystery.

    I like the mystery, and I hope we never give up trying to understand it, or at least move us on to the next mystery we can't understand.

    Thanks.
     
  20. May 12, 2006 #19

    DrChinese

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    As ZapperZ says, there is nothing more here than the superposition of entangled particle states. EPR/Bell/Aspect shows that the predicted behavior occurs even when it might otherwise be counter-intuitive. Thus the question becomes, why is there superposition in the first place? And why does this make you *think* that something is occurring non-locally?

    As you will learn, there is a lot of debate about the nuances of this, and a lot of it is semantic differences. There really is no one absolute correct answer that will please everyone. But ZapperZ's point is worthy: why is there superposition, and why is there something like the Heisenberg Uncertainty Principle? Once you accept these starting principles, everything else follows.

    As to a good reference to a more recent Aspect-like experiment that is well known: Violation of Bell's inequality under strict Einstein locality conditions. This experiment was performed over a half kilometer of separation.
     
  21. May 12, 2006 #20
    DrChinese, I have read about that experiment. Thanks.

    A bigger mystery is how simple organic beings can even have such conversations, let alone understand as much as we do.

    I wax philosophical, forgive me....

    Daryl
     
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